1. Field of the Invention
The present invention relates to switching mode power converter circuits. More particularly, the invention relates to single-ended, isolated switching mode power supplies that utilize secondary side, self-driven synchronous rectification.
2. Description of Related Art
Switching mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switching mode converter provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. An isolated forward converter is a particular type of switching mode converter in which an input DC voltage is periodically switched across the primary side of a transformer using one or more power switches, typically provided by MOSFET devices. The transformer provides isolation between the voltage source on the primary side and a load on the secondary side. Two diodes on the secondary side rectify the switched and isolated input voltage, including a forward diode connected in series with secondary winding that conducts current to the load when the input voltage is present across the secondary winding and a free-wheeling diode connected in shunt with the secondary winding that conducts current to the load when the input voltage is not present across the secondary winding.
In order to improve the efficiency of the forward converter circuits, it is known to replace the rectifying diodes with power switches (e.g., MOSFET devices). The operation of the power switches may be controlled so that they are turned on and off in synchronism with the switched input voltage. The control signals applied to the power switches must be synchronized as closely as possible to the current inflection points of the output inductor. This synchronous rectification provides higher efficiency over the foregoing forward converter configuration since the forward voltage drop of the power switches is much lower than that of diodes.
So-called active-clamp forward synchronous rectification topologies are very popular in the telecommunication power business. The topology could be found in the power applications ranging from 50 W to 300 W. Power systems for telecommunications applications must satisfy certain considerations with respect to size and protection of the secondary side synchronous rectification MOSFETs. Isolated telecommunication power modules are typically designed to achieve a minimum size. Thus, a simple circuit is usually preferred over a more complex circuit.
Nevertheless, this simplicity of the power converter circuit is difficult to achieve. In an isolated power converter topology, it is necessary to provide bias voltage to the synchronous rectifying MOSFETs. This bias voltage is often provided by a peak charging circuit that is coupled to an additional secondary winding of the transformer, which tends to increase the size and complexity of the transformer winding structure. In addition to the bias circuitry, it is known to control the power switches using a self-driven topology in which the gates of the synchronous rectifying MOSFETs are coupled to the ends of the power transformer secondary winding. The MOSFETs are therefore activated synchronously with the changing voltage across the secondary winding. A drawback with the self-driven topology is that the operation of the power switches can not be controlled to regulate the output voltage of the power converter. Moreover, the voltage applied to the gates of the MOSFETs from the secondary winding may damage the MOSFETs. In view of these drawbacks with self-driven topology, a controlled-driven topology is known in which a separate control circuit is used to detect the current inflection points and activate the MOSFETs. But, a drawback of the controlled-driven topology is that the control circuit increases the complexity and cost of the power converter. Since it is desirable to reduce the size of power converter modules, such as for telecommunications applications, the increased complexity of the power converter due to the bias circuitry as well as the MOSFET control circuit represent significant impediments to providing a simple module structure.
Accordingly, it would be desirable to provide an isolated forward converter power module with synchronous rectifier control and bias circuitry that overcomes these drawbacks of the prior art.